There are numerous advantages to implementing a radio communication system using digital techniques. Notably, there is enhanced system capacity, reduced noise, and reduced hardware and associated power consumption. There has been proposed several digital radio communication systems. For example, there is shown and described in the commonly assigned U.S. Patent Application entitled "Multi-Channel Digital Transceiver and Method" filed on even date herewith, the disclosure of which is hereby expressly incorporated herein by reference and of which the Applicant is a co-inventor, several preferred embodiments of radio communication systems implementing digital techniques.
Fundamental to the digital radio communication system is the requirement that the received analog radio signal be digitized. The well known Nyquist criteria provides that such digitization is accomplished with minimal error at about twice the bandwidth of the analog signal. In U.S. Pat. No. 5,251,218 a methodology typical of the prior art is disclosed for digitizing an analog radio frequency signal in accordance with this principle. It will be appreciated, however, where the radio signal occupies a large bandwidth, ADCs capable of operation at very high sampling rates are required. Such devices, to the extent they are available, are expensive and often suffer reduced performance, i.e., have significant distortion and increased power consumption when operated at high sampling rates.
The spectrum allocated to radio communication systems is typically large with respect to the requirements for digitizing. In some radio communication systems, however, although the desired signal occupies a large bandwidth, not all of the bandwidth is occupied by signals of interest. In cellular radio telephone communication systems, for example, the communication bandwidth is not contiguous. The cellular A-band, for example, is allocated a bandwidth of 12.5 megahertz (MHz). Spectrally, however, the entire A-band covers 22.5 MHz of bandwidth in two discontinuous portions. In order to digitize the A-band, one would need an ADC capable of operating, according to Nyquist criteria, at least at 45 MHz or 45 million samples per second (Ms/s), and more reliably at 56 Ms/s.
Therefore, there is a need for a device for digitizing wideband frequency band signals which is does not require high sampling rates, and does not significantly increase the amount of hardware required for the communication system.